In an operational amplifier circuit the slew rate is a measure of the variation in the output voltage in terms of volts per unit time. The higher the slew rate, the quicker an output voltage of the operational amplifier can reach a desired target voltage value. It is therefore desirable to maximize the slew rate in an operational amplifier circuit.
Two different types of approaches have been taken to increase the slew rate in single state operational transconductance amplifier (OTA) circuits. The first approach has been to provide an additional differential input stage that increases the bias current of the operational amplifier in order to enhance the slew rate. This type of approach is referred to in this document as a “Bias Boost” approach. An example of the Bias Boost approach is described in a paper by R. Klinke and B. J. Hosticka entitled “A Very-High-Slew-Rate CMOS Operational Amplifier,” IEEE Journal of Solid-State Circuits, Volume 23, No. 3, pp. 744-746 (June 1989).
FIG. 1 illustrates the Bias Boost principle applied to an operational amplifier 110. As shown in FIG. 1, a dynamic bias circuit 120 monitors the signals at the input of operational amplifier 110. When a large differential signal is detected, the dynamic bias circuit 120 temporarily increases the bias current to enhance the slew rate. The static bias circuit 130 provides the bias current during the small signal operation. The static bias circuit 130 determines the small signal properties of the operational amplifier 110 when the dynamic bias circuit 120 is not operational. Only the presence of large differential signals at the input can turn on the dynamic bias current from dynamic bias circuit 120.
The Bias Boost approach necessarily requires extra circuitry. It is sometimes very difficult to prevent the extra circuitry from degrading the electrical characteristics of the underlying operational amplifier. Another example of the Bias Boost approach is described in a paper by S. Baswa, A. J. Lopez-Martin, R. G. Carvajal, and R. Ramirez-Angulo entitled “Low-Voltage Micropower Super Class AB CMOS OTA,” Electronics Letters, Volume 40, pp. 216-217 (February 2004).
The Bias Boost method is not desirable for two stage CMOS amplifiers. The Bias Boost method is not compatible with some commonly used operational amplifier architectures. For example, boosting the tail current only serves to enhance the slew rate in one direction in a folded cascode amplifier. The folded cascode amplifier is the basis for almost all low power amplifiers because (1) it is very easy to add a buffer stage for driving large capacity loads, (2) it has a very high gain, and (3) it is relatively easy to add a secondary complementary differential input stage for rail to rail operation.
The second approach has been to add current directly into the load in order to help the operational amplifier drive the output capacitance. This type of approach is referred to in this document as a “Load Boost” approach. An example of the Load Boost approach is described in a paper by K. Nagaraj entitled “CMOS Amplifiers Incorporating A Novel Slew Rate Enhancement Technique,” IEEE 1990 Custom Integrated Circuits Conference, pp. 11.6.1-11.6.5 (1990).
The increased slew rate capability is provided by an auxiliary circuit that is automatically activated during fast signal transitions. The slew enhancing currents are applied directly to the output and not to the bias of the main amplifier. This approach avoids the problems that result if the main amplifier has to handle large transient currents. FIG. 2 illustrates an illustrative prior art Load Boost circuit 200 for applications that present relatively small capacitative loads but require very high slew rates.
The Load Boost method is not desirable for two stage CMOS amplifiers. The Load Boost method is deficient because positive feedback into the amplifier's output can easily lead to oscillations if the slew detect circuit is too slow or has a differential trigger voltage that is too small. Even when properly compensated, the Load Boost method can lead to excessive overshoot and excessive recovery time. This means that the Load Boost method is a very uncontrolled manner with which to drive the load.
For the reasons described above, the prior art approaches are not desirable for use in two stage CMOS amplifiers. Therefore, it would be advantageous to have a more efficient system and method for providing slew rate enhancement in two stage CMOS amplifiers.
Before undertaking the Detailed Description of the Invention below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation, the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like.
The term “controller” means any device, system, or part thereof that controls at least one operation. A controller may be implemented in hardware, software, firmware, or combination thereof. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely.
Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior uses, as well as to future uses, of such defined words and phrases.